This invention was conceived in response to a drilling problem encountered by ODP. The ODP (Ocean Drilling Program) is an international scientific project which bores into the ocean floors and retrieves cores for geologic research. Some areas of ocean floor lava flows have been very difficult to bore and core into because of the brittle hard and broken nature of the deposits. The walls of boreholes formed in this material are very unstable. The borehole walls would almost always collapse and fill the hole whenever the drill pipe and bit were removed. Sometimes the walls would collapse on the drill pipe and bit permanently locking them in the formation. In this situation cores and samples have not been obtained below the depth which one bit can penetrate. In order to penetrate deeper in these formations it was decided to drill a first bit and drill pipe into the formation, detach from the drill pipe and leave it embedded in the formation for use as a conduit or cased hole. A smaller second drill string and bit will then pass through the previous ones and drill into deeper sections of the formation. The first bit had to permit a four inch diameter bit to pass through it and the overall diameter of the first bit had to be kept to a minimum. Several concepts for the first bit have been suggested including both fixed cutter and rolling cutter designs. One of the rolling cutter concepts is an embodiment of this invention.
As mentioned earlier it is often necessary to trip the drill pipe out of oil wells to permit testing of various sorts. These tests include taking cores, taking fluid samples, measuring formation pore pressure, taking electric logs etc . . . Many of these tests are accomplished with tools which are lowered into the hole and retrieved by means of cables or wireline systems. The wireline systems can move the tools in and out of the holes at several hundreds of feet per minute. Tripping drill pipe usually requires about one hour per 1000 ft. of depth. The cost per hour of land drilling rigs averages from 300 to 500 dollars per hour and for offshore rigs the averages are from 1000 to 2500 dollars per hour. Therefore on a 10,000 ft. oil well the trip cost alone for testing can range from 3,000 to 25,000 dollars. There is a need for oil field bits which can perform competitively with current bits and permit certain downhole testing without having to be tripped out.